<!DOCTYPE html>
<html>
<head>
    <title>Legacy GH-16 iframe</title>
</head>
<body>
<button id="replaceSrc">Replace iframe source via top window</button>
<script>
    document
        .getElementById('replaceSrc')
        .addEventListener('click', function () {
            window.top.document.getElementById('iframe').src = 'iframe.html';
        });
</script>
</body>
</html>